Black Hole-Resistant Planet [DOCTOR WHO-RELATED]

[Tex]

So I'll come out right now and say that I am a really faithful Whovian.

I've just watched Series 2: Episode 9: The Impossible Planet. In this episode (I won't give out spoilers), the Doctor and Rose find a planet that is holding its own against a black hole hardly an asteroid's throw away. What I'm wondering is if you guys think this is possible or not.

The episode explains that the planet is beaming out a huge gravity field in the shape of a funnel into clear space. This is how a group of unnamed humans came to temporarily reside on this planet, by simply flying through it. Somehow, an unnamed object deep within the planet's core is keeping it more or less unaffected by the gravity of the black hole. It is, however, still being pelted with huge amounts of debris from the wreckage of structures the black hole has ripped apart.

Do you think, somewhere in our universe or another one, this could possibly happen?

[Dominatus]

I seriously doubt it. Although I'm certainly no expert, assuming the funnel thing was even possible, it would require insane amounts of energy to remain suspended like that. In fact, so much energy would be needed that it would show up on scans at incredible distances- assuming it were still the Milky Way, we may even be able to detect it on earth, and wonder what could create such a strong energy signature.

And being in such close proximity to the black hole, it wouldn't be enough to simply "resist" the incredibly strong gravitational pull of said black hole. Odds are, the entire asteroid/ planet would be torn apart by the gravitational forces pulling in one direction and the anti gravity field resisting in the opposite.

Besides, there are examples of incredibly dense neutron stars being "consumed" as they fall into the black hole. If something with such high mass and gravity cannot even resist the gravity of a black hole, much less counteract it, then I find it highly improbable that any kind of technology can exist that would resist it.

Of course, this is loosely based on what I know, and is part fact, part conjecture, and should be taken with a grain of salt. Feel free to correct me on any if this, I'm sure I got something wrong.

[AndrewBCrisp]

It's been some years since I've watched that episode, but one thing I remember was how much the characters were OVERstating the danger.

Black holes, really, are like any other lump of matter re: the gravity fields they put out and how objects can orbit them. The difference comes when you get too close to a black hole. IIRC - someone with more physics experience can confirm or refute this - "too close" is generally defined as 1.5 x the black hole's radius. Outside that distance, orbital and escape velocities are huge, but still below the speed of light. At 1.5 radii, the orbital velocity equals light and as you go closer still, orbital and escape velocities exceed light - nothing can orbit there.

Given the size of the black hole as depicted in the epsiode, the "Impossible Planet" was not impossible at all. Of course, said planet would likely be whipping around the hole at a good clip, so rendezvous and escape would probably have insane delta-V requirements. Perhaps, then, the "Impossible" part resides in the fact that going there and returning would require so much energy that few could afford to do so.

(Also: hawking radiation on a planet orbiting a black hole would likely be immense, so survival on the side facing said hole would be... questionable. But then, this is Doctor Who, where if you go far enough in the future, any and every magical effect is handwaved as sufficiently advanced technology).

Given that the planet's real purpose is <REDACTED>, then it makes more sense for the planet to sit inside the 1.5 radii distance for its orbit, at which point some form of near-magical technology would be needed to keep it from falling in. If the planet fit's the IAU's definition, however, it would likely be too big to say there for long before tidal forces tear it to pieces. Something smaller - a space station, maybe, or some kind of pod - would serve the same purpose and survive, again, given near-magical technology to keep it in orbit. Getting to - and escaping from - the planet reaches the realm of the physically impossible. Which serves it's real purpose of <REDACTED> just fine.

Those are my thoughts, based mostly on my 5-6 year old memories of the episode, and my memories of physics, so expect errors. A similar situation was dealt with in a short story published in the June 1994 Analog Science Fiction and Fact, called "In the Dazzle" by Robert R. Chase - well worth a read.

[Warhorse]

Just how far away is the black hole from the planet, anyway? If it's more than, say, a million miles or so, then the planet is in no danger whatsoever from the hole's gravity, although it might well get fried by xrays if enough stuff is falling into the hole.

[K^2]

Orbital mechanics in proximity of a black hole is pretty straight forward, actually. Circular orbits with r > 3r_s are stable. In other words, you can have an object orbiting a black hole just an event horizon's diameter away from the same. For a small black hole, we could be talking about tens of kilometers.

Of course, if we are talking about a planet, its orbit can't be closer to the black hole than the size of the planet. There are also tidal forces to consider. There is going to be an equivalent of the Roche limit within which any planet would get torn up and become part of the accretion disc. So something planet-sized would have to be placed a bit further away, but it can still be very close.

Without knowing a bit more about the specifics, like the size of the planet, its composition, and distance from the black hole, I couldn't really tell you for sure whether such a thing is going to be possible. But in general, it's entirely plausible.

"too close" is generally defined as 1.5 x the black hole's radius

There are circular orbits that close in, but they are not stable. The object in orbit would require some station-keeping. Plausible for a ship, but not for a planet. Stable orbits start at 3x the radius.

Edited December 9, 2013 by K^2

[Skyler4856]

It depends on the mass of the black hole. Apparently black holes are produced daily at the LHC yet we are still fine

[K^2]

Apparently black holes are produced daily at the LHC yet we are still fine

What gave you that idea?

[Danish_Savage]

They make singularities there quite often. You could define them as black holes, as their volume is incredible little compared to their mass. The mass however is a few units, and they collapse almost immediately.

[Skyler4856]

Sorry my mistake singularities not black holes

[K^2]

They make singularities there quite often. You could define them as black holes, as their volume is incredible little compared to their mass. The mass however is a few units, and they collapse almost immediately.

That's absolute nonsense. At energies of 7TeV, which is the most you get per nucleon at LHC, the corresponding Schwarzschild Radius is on the order of 10^-50m. That's 15 orders of magnitude smaller than Plank Length. Let alone nucleon size, which is on the order of femtometers.

There is nothing even remotely close to a singularity being produced at LHC.

[Idobox]

From memory, the planet is not orbiting, is extremely difficult to access, and they go because they wonder what kind of technology provides the humongous power to keep it from falling. The planet can be seen from very far.

I also remember stuff being destroyed by tidal forces pretty close from the planet, but closer to the black hole, including stars, but I 'm not sure of my memories on that point.

The planet is held by some form of anti gravity, we have no idea how that works, so there is no reason to believe there are any significant tidal forces within the funnel. But it's also important we're talking of doctor Who, the series that explains the plot using timey-wimey ball of , er, stuff. It's not science-fiction, it's not even future fantasy, it's a glorious acid trip disguised as cheap sci-fi, and pretty much nothing in it is remotely plausible. And that's why it's awesome.

So, with our current understanding of physics, no way. With some form of antigravity, why not?

[K^2]

From memory, the planet is not orbiting, is extremely difficult to access, and they go because they wonder what kind of technology provides the humongous power to keep it from falling. The planet can be seen from very far.

I also remember stuff being destroyed by tidal forces pretty close from the planet, but closer to the black hole, including stars, but I 'm not sure of my memories on that point.

That's useful information. Tidal forces can be manageable for a large enough black hole, but the force required to keep an object stationary diverges as you approach the horizon. So yeah, there is no way to hold a planet in place as far as we know.

[Comrade Jenkens]

That whole episode annoyed be as anything can orbit a black hole just like it could a star!